1. Field of the Invention
The invention relates to silane-terminated hybrid polymers based on polydiorganosiloxane-urethane copolymers having moisture-curing silane end groups, to a process for preparing them, and to their use.
2. Description of the Related Art
The properties of polyurethanes and silicone elastomers are complementary within wide ranges. The combination of both systems ought therefore to give access to materials having excellent properties. Polyurethanes stand out by virtue of their good mechanical strength, elasticity, and their very good adhesion and abrasion resistance. Silicone elastomers, on the other hand, possess excellent temperature, UV, and weathering stability. They retain their elastic properties at relatively low temperatures and therefore also have no tendency toward embrittlement. In addition they possess special water repellency and anti stick surface properties.
The combination of the advantages of both systems leads not only to new compounds having low glass transition temperatures, low surface energies, enhanced thermal and photochemical stabilities, low water absorption, and physiologically inert materials but also to materials in which the properties can be adjusted by the proportion and chemical composition of the silicone and polyurethane constituents.
Investigations have been conducted in order to overcome the poor phase compatibilities of the two systems. Producing polymer blends allowed sufficient compatibilities to be achieved only in a few specific cases. Not until the preparation of polydiorganosiloxane-urethane copolymers described in J. Kozakiewicz, Prog. Org. Coat., 1996 (27), 123 and of polydiorganosiloxane-urea copolymers, in I. Yilgör, Polymer, 1984 (25), 1800 was it possible to achieve this objective. The reaction of the polymer building blocks takes place latterly in accordance with a comparatively simple polyaddition, such as is employed for the preparation of polyurethanes. Starting materials used for the siloxane units are hydroxyalkyl-terminated polysiloxanes (siloxane-urethane copolymers) and aminoalkyl-terminated polysiloxanes (siloxane-urea copolymers). These form the soft segments in the copolymers, analogous to the polyethers in straight polyurethane systems. As hard segments the customary diisocyanates are used, and may also be modified by addition of short-chain diols, such as 1,4-butanediol for the purpose of achieving higher strengths. The reaction of the amino compounds with isocyanates is spontaneous and as a general rule does not require any catalyst. The reaction of hydroxy compounds is carried out by addition of catalysts—mostly tin compounds.
The silicone and isocyanate polymer building blocks are readily miscible within a wide range. The mechanical properties are determined by the ratio of the different polymer blocks (soft silicone segments and hard urethane/urea segments) and substantially by the diisocyanate used. In the case of the urea copolymers almost exclusively thermoplastic materials are obtained. As a result of the strong interactions of the hydrogen bonds between the urea units these compounds possess a defined melting or softening point. These interactions are much less pronounced in the urethane copolymers, and so these materials generally form highly viscous liquids.
For elastomers, seals, adhesives and sealants or antistick coatings, conventional polysiloxanes are employed in the form of thixotropic pastes. In order to attain the desired ultimate strengths, various means of curing the compositions have been developed, with the objective of fixing the desired structures and adjusting the mechanical properties. Generally, however, the polymers require blending through the addition of reinforcing additives, such as highly disperse silicas, for example, in order to attain sufficient mechanical properties.
In the case of the curing systems a distinction is made essentially between high-temperature-vulcanizing (HTV) systems and room-temperature-vulcanizing (RTV) systems. In the case of the RTV compositions there are both one-component (1K) and two-component (2K) systems. In the 2K systems the two components are mixed and thereby catalytically activated and cured. The curing mechanism and the catalyst required may differ. Curing is normally effected by peroxide crosslinking, by hydrosilylation by means of platinum catalysis or by silane condensation. A compromise has to be made between working time and cure time. The 1K systems cure exclusively by silane condensation with the ingress of atmospheric moisture. This curing mechanism is of interest for simple processing of the materials, as needed for adhesives and sealants. In the absence of moisture, the 1K systems are stable on storage for prolonged periods of time. Curing takes place primarily by way of condensation-crosslinking alkoxy-, acetoxy- or oximo-terminated compounds.
Conventional polyurethanes are employed analogously as with 1K systems or 2K systems. The 1K compositions cure by the contact of isocyanate-containing prepolymers with the atmospheric moisture. In this reaction the isocyanate group is broken into an amino group and carbon dioxide. The amino compound formed reacts immediately with further isocyanate. In the case of spray foams the carbon dioxide released is desirable for the generation of foam alongside the propellant gas, but in adhesives and sealants can lead to problems in application as a result of bubbling. A further disadvantage is the general need to set low viscosities for favorable processing. Since the polyurethane prepolymers usually exhibit very high viscosities, either processing must be carried out at relatively high temperature or monomers or short-chain oligomers must be added to the polymer in order to lower the viscosity.
A critical point is that the remaining free isocyanate groups, owing to their high reactivity, can also exhibit extreme irritant and toxic effects. Additionally, some amines which are formed from the monomeric isocyanates are suspected of being carcinogenic. Consequently, a residual monomer content or the addition of monomers may in future be problematic from standpoints of toxicology.
A multiplicity of investigations, therefore, are also concerned with the preparation of isocyanate-free polyurethane prepolymers. One very promising approach which has emerged is the preparation of silane-crosslinking prepolymers. In this case, isocyanate-containing prepolymers are generally reacted with aminosilanes. This produces silane-terminated polymers, in which the silane groups may carry further reactive substituents which crosslink on ingress of moisture, such as alkoxy, acetoxy or oximato groups. These materials are then cured in analogy to the silicones described above. With these polymers, however, it is necessary to obtain a compromise between the mechanical properties, determined largely by the molecular weight, and the associated viscosity. High molecular weights are important for good tensile strengths, but these systems possess very high viscosities and can be processed only at relatively high temperatures or as a result of the addition of solvents or plasticizers.
The mechanism of curing by silane condensation is known for polydiorganosiloxane-urea copolymers as well and is used for example in WO 96/34030 and EP-A-250 248 to produce special antistick coatings. Polymers of this kind can be synthesized only in solution or by coextrusion, since almost without exception the copolymers obtained are solid and possess thermoplastic properties prior to crosslinking. As a consequence they can be processed only at relatively high temperatures or by application from solution.
WO 95/21206 describes polyurethane copolymers which are terminated with hydrolyzable silanes and are prepared by reacting an isocyanate-terminated polyurethane-silicone-polyether prepolymer with diamine chain extender and amino silane in aqueous dispersion. These polyurethane copolymers can be processed only as a dispersion. The presence of water gives rise in numerous instances to processing problems, in connection with blending with water-sensitive formula constituents, for example, or else impairments during application, by shrinkage after drying.